The present invention relates to a method of treating an epoxy resin coated metallic substrate, such as a ship's hull, with an epoxy-silicone-adhesive paint as an intermediate tie-layer to effect the bonding of subsequently applied duplex silicone foul release coatings.
Metallic structures submerged in sea water, such as ship bottoms, are generally infested with organisms, such as barnacles, tube worms, and algae, which can become attached to the surface of these structures causing increased fuel consumption due to increased drag. Routinely, anti-fouling paints are used to treat the surface of these exposed substrates to minimize corrosion. Silicones have been known as effective anti-fouling coatings as early as the 1970's, as shown by U.S. Pat. Nos. 4,025,693, 4,080,190 and 4,227,929.
In Griffith, U.S. Pat. No. 5,449,553, which is incorporated herein by reference, a nontoxic anti-fouling system is described which involves the application of a silicone room temperature vulcanizable (RTV) condensation cure composition, for example, GE RTV 11, as a topcoat, which interfaces with sea water, and which is anchored onto a semi-cured organo-silicon condensation cure RTV bonding layer. An example of a semi-cured RTV condensation cure bonding layer is Silgan J-501 of the Wacker Silicones Corporation of Adrian, Mich., as shown in the aforesaid Griffith, U.S. Pat. No. 5,449,553. The semi-cured RTV condensation cure bonding layer is directly applied, and is in contact with the epoxy treated metallic substrate, such as a ship's hull, and can serve as an anchor for a subsequently applied silicone condensation cure RTV topcoat.
The combination of these condensation cure RTV's has been found to be effective as a duplex non-toxic silicone foul release system when properly secured to a ship's hull. The application of the duplex non-toxic silicone foul release system is preferably made under ambient conditions onto an epoxy-coated metallic substrate, for example a ship's hull, which has been thoroughly cleaned, sand blasted to metallic substrate, and then freshly coated with an epoxy resin.
Experience has shown that even though adhesion between the silicone room temperature vulcanizable (RTV) composition which interfaces with sea water, and the aforementioned silicone-organic bonding layer in the dual silicone foul release system is generally satisfactory, adhesion between the silicone bonding layer and the epoxy coating on the metallic substrate is often unsatisfactory.
A procedure which has been developed to improve adhesion between the silicone-bonding layer and the epoxy resin coating on a metallic substrate, such as a ship's hull, is based on the use of an equal parts mixture of butanol and a curable epoxy resin mixture, referred to sometimes as "epoxy mistcoat". In making the epoxy mistcoat, there is used in combination with butanol, an epoxy resin paint, which may contain Epon 828 resin and an amine curing catalyst for the epoxy resin, such as VERSAMID 140 of the Henkel Coporation of Ambler, Pa. As a result of applying the epoxy mistcoat directly onto the epoxy-coated ship's hull, a binary epoxy layer is formed prior to the application of the duplex non-toxic silicone foul release system. While an improvement in adhesion has been found to occur between the resulting binary epoxy layer, and the subsequently applied silicone-organic bonding layer, considerable environmental concerns have been recognized as a result of butanol emissions. In addition, special mixing procedures, and time restrictions have been found necessary to achieve satisfactory bonding between the epoxy mistcoat and the silicone-organic bonding layer.
Additional techniques are therefore needed to provide an environmentally safe and efficient method to satisfactorily anchor the duplex silicone foul release coating system onto an epoxy-coated ship's hull.